The present invention relates to a method for de-ashing and transportation of coal. More particularly, the present invention relates to a method for crushing, pelletizing, and de-ashing coal at coal mines and then transporting coal through slurry pipeline systems or bulk shipments by trains, trucks, conveyors, etc.
The known methods for coal shipments include (1) slurry transportation in which pulverized coal is slurried in water and transported through pipelines and (2) bulk shipments in which coal is handled in the form of lumps.
According to the slurry transportation method, coal is pulverized to an average particle size of about 0.1 mm and slurried in water and the coal slurry is transported through pipelines to the place of consumption. This method, however, has the following disadvantage. In order for the slurry to be stable enough to prevent coal particles from settling in the pipeline, the slurry should contain about 20% of fine coal particles less than 40 microns in size. If slurry is therefore made containing fine coal particles, by this the slurry can be made having an improved stability, but dewatering of the slurry is then made difficult as a matter of course, so that it is unavoidable in this case to carry out a powerful dewatering step at the destined place of consumption: For example, a slurry transported through a pipe line is put for a solid-liquid separation by a centrifugal separator, but there still tends to remain fine particles suspended in the separated liquid, so that it further is operated to pass the separated liquid through a thickener and to further refine it through a waste-water refining apparatus.
As pointed out above, to directly slurry fine coal particles involves the serious shortcoming of being accompanied by an extreme difficulty for dewatering.
On the other hand, the shipping of lump coal requires troublesome, costly loading and unloading and measures for environmental pollution with dust and spontaneous combustion during storage.
Coal contains rocks and soil, and transporting them altogether is uneconomical. To solve these problems, the following methods have been proposed. (A) Removing rocks from coal by gravity separation. (B) Adding fuel oil or light oil to the coal slurry at the terminal of a pipeline system, whereby performing de-ashing and pelletization based on the principle of oil agglomeration in water (abbreviated as OAW hereinafter). (C) Adding water to a coal-oil mixture, whereby removing ash caught by water.
The method (A) requires a coal dressing plant with additional cost for equipment and labor, and yet has the problem unsolved in the dewatering of slurry.
The method (B) has inherent demerits mentioned below.
(1) It is impossible to control the OAW operating conditions according to the fluctuation of coal rank. The rank of coal fluctuates even in the same coal mine, and the ash content and lipophilic property of coal vary accordingly. OAW utilizes the lipophilic property of coal, and the oil to be added should be controlled according to the rank of coal. However, this is impracticable in view of the fact that tens of thousand tons of coal is flowing in the pipe at all times. Even though the fluctuation of coal rank is found at the entrance of the OAW facility, it is impossible to cope with the time lag. As the result, slurry water is discharged, with agglomeration remaining incomplete. This might lead to water pollution with fine coal particles.
(2) It is impossible to add oil in a proper quantity according to the pulverization of coal that occurs during slurry transportation. The quantity of oil to be added in the OAW process should be changed according to the particle size of coal powder, and the more the fine powder, the more the oil required. The ratio of pulverization varies depending on the rank of coal, but it is very difficult to estimate it beforehand.
(3) The OAW process requires a great deal of powder and time and an apparatus of infeasibly large volume. For instance, if five million tons of coal is to be transported per year, the OAW apparatus would have to have a flow rate of 2000 m.sup.3 /h, assuming 30 wt % solids in the slurry. Such an apparatus would be 1000 to 2000 m.sup.3 in volume and require 4000 to 12000 kW for agitation, assuming a power consumption of 2 to 6 kW.h for 1 m.sup.3 of slurry, according to the inventors' calculations.
(4) The OAW process, which is intended to separate pure coal by agglomeration from ash in pulverized raw coal, has an inherent demerit that it cannot separate ash particles enclosed by pure coal. This is the reason why the de-ashing ratio was 30 to 40% with the conventional OAW process.
Finally, the method (C) is not economical because the oil used more than 50 wt % based on coal is eventually burned. Using oil in such a large amount goes against the times when replacement of petroleum by coal is being advocated.